Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called “membrane-electrode-assembly” (MEA) comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, having very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles.
The membrane-electrode-assembly is sandwiched between a pair of electrically conductive porous fluid distribution media layers. The MEA together with the fluid distribution elements form a compliant layer, which is then sandwiched between a pair of electrically conductive contact elements that serve as current collectors for the anode and cathode, and may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 & O2/air) over the surfaces of the respective anode and cathode.
Bipolar PEM fuel cells comprise a plurality of the membrane-electrode-assemblies stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar or separator plate. The separator or bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, where each bipolar plate electrically conducts current between the adjacent cells. Conductive contact elements at the ends of the stack are referred to as end or terminal separator plates. These terminal separator plates contact an MEA along one side and are adjacent to an end plate of the stack, and collect current and transport gases along a single side (rather than both sides of a bipolar plate). The separator plate elements of the stack thus serve as an electrically conductive separator element between two adjacent cells, and typically have reactant gas flow fields on both external faces thereof, conduct electrical current between the anode of one cell and the cathode of the next adjacent cell in the stack, and have internal passages therein through which coolant flows to remove heat from the stack.
The PEM fuel cell environment is highly corrosive, and accordingly, the separator plates and the materials used to assemble them must be both corrosion resistant and electrically conductive. Bipolar plates are generally fabricated from two separate conductive plates, and may be constructed of electrically conductive metal or composite materials. These individual plates are joined together to form an interior formed between the plates which contains cooling passages. The plates must withstand the harsh conditions of the fuel cell, while providing high electrical conductivity, low weight to improve gravimetric efficiency, and durability for long-term operational efficiency. Further, separator plate contact with adjacent elements must be optimized to enhance fuel cell operation. There remains the challenge to optimize these electrically conductive elements and assemblies made therefrom in a fuel cell to promote efficiency as cost-effectively as possible.